This invention relates to an apparatus and a method of its utilization in determining the symmetry of a surface.
In a preferred embodiment, the invention relates to an apparatus and method for determining the symmetry of a light reflecting surface by superimposition and comparison of a first reflected image upon the same image ("second reflected image") which has been rotated in a common plane about a common axis relative to the first reflected image; wherein such apparatus and method find preferred application in keratometers and in the art of keratometry.
For purposes of background in the field of keratometry, the reflection of a symmetrical image, such as, a circle, upon a cornea will be distorted to an eliptical image if the surface examined is not truly spherical. The distortion is a consequence of an astigmatism. However, it really is quite impossible, or at least not reproducible, to judge the nature of the astigmatism by viewing the reflected image, per se. But, if the departure from circular symmetry is read against a reference, information describing the astigmatic surface can be obtained. One of the most telling techniques involving the use of a reference is to compare the reflected circular image with itself, albeit rotated 90.degree. and, ideally, superimposed upon the first reflected image. If the cornea section under view is astigmatic, the image of the circular light source will be eliptical and by superimposition of a second image which has been rotated 90.degree. relative to the first image upon the first image, the respective lesser and major axes of the resulting ellipses will be orthogonal, and the observable departure from circularity is easily detected and quantitative measurement of the corneal defect is possible.
Keratometry is routinely employed to diagnose and define corneal astigmatism and to define corrective lenses, but is also employed during eye surgery--particularly during final suturing to minimize already existing or potentially induced astigmatism by local suture adjustment.
Conventional keratometers, however, are expensive, awkward to use, and consequently, their potential usefulness during surgery has never been realized.
U.S. Pat. No. 4,355,871 issued Oct. 16, 1982, to Nevyas and Traub is generally representative of prior art which recognizes the basic approach as described above relative to keratometers in keratometry; and is specifically representative of the technology relative to rotation and superimposition of eliptical image to detect astigmatism. However, there are practical limitations in the operation of devices such as that described in the '871 patent which limit their usefulness. These limitations relate to the complexity and the number of optical elements required to transform the image and read its information relating to symmetry. For example, the use of optical elements such as multiple beam splitting devices significantly decreases the light intensity ultimately made available to the observer and a functional dependence of the position of the optical device, i.e., its orientation about the axis of observation, on the degree of rotation of the second reflected image limits the utility of prior art devices such as those described in the '871 patent. Nevertheless, for purposes of the present disclosure, said U.S. Pat. No. 4,355,871 is fully incorporated herein by reference to the extent that it provides a fully enabling description of keratometric methods and devices which depend upon the reflection of a symmetrical target, such as a circular pattern, upon the cornea and the transmission of the reflected image to a point of convenient observation, such as, the ocular of a microscope in the surgical theater; wherein said image has superimposed the identical image which has been rotated, relative to the first image, a preferred angle of 90 degrees. Such a visual comparison of the two images gives an immediate qualitative interpretation of the lack of spherical symmetry of the cornea. This information can be interpreted quantitatively as well by the use of reading instruments which compare the reflected images against measuring grids, for example. Relative to the apparatus and method described in the '871 patent, it is further noted that such relevant prior art devices are further complicated by the requirement that optical elements account for any path length differences experienced by the primary image light ray family relative to the distance traversed by the secondary light ray family of the reflected, rotated image. Additionally, it is noted that such prior art devices are plagued with a left-right image reversal which requires user accommodation; although the prior art teaches that such limitations can be avoided, such limitations are avoided only at the expense of additional intensity losses and the addition of further elements to the already complicated system.
Thus, while the present invention will, for convenience of dislcosure, be recognized as an improvement over the prior art device exemplified in the fully incorporated by reference '871 patent, the present invention relies on a novel optical system which comprises only three elements for the desired comparison of a first and a second reflected image wherein the second image has been rotated 90 degrees and superimposed on said first image. Besides simplicity, robustness, and advantages of cost and maintenance, the optical system of the present invention is further distinguished over the prior art in the following ways: 1.) The optical system achieves the desired 90 degree rotation independently of the rotational attitude of the reflecting surface or the instrument; 2.) The final composite image does not suffer from left to right reversal and there is no need for conscious adjustment toward equalization of the light path of the first image relative to the light path of the second image; 3.) Because of the reduced number of optical elements in the optical system, the two images are presented at fully one-half of their original intensity; this result is achieved, principally, by having a single beam splitting means which is commonly used by both light paths; and 4.) The optical system of the present invention is sized to be fitted into the body of conventional binocular microscopes without exceptional costs, modifications or compromise of other functions of the microscope.